Offshore constructions typically comprise a platform, e.g. a rig that has its position stabilized by a plurality of anchoring elements. The platform is connected with respective mooring lines to the anchoring elements. The mooring lines typically extend from a fairlead on respective corners of the platform to their respective anchoring elements on the seabed. The portion of the mooring lines in the vicinity of the platform is still close to the sea surface, but may be overlooked by passing vessels. Therewith a risk exists that these passing vessels collide with a mooring line which may result in damages to the mooring line or the vessel. Also the stability of the platform may be jeopardized by a displacement of the anchoring element due to forces acting thereon as a result of this collision. In attempting to mitigate this risk, warning zones are defined associated with the shallow parts of the mooring lines and position data of passing vessels is monitored, e.g. by a radar system. If it appears that a monitored position is inside a warning zone, an alert message is generated. Upon noticing the alert message, platform personnel can order the commander of the vessel to maneuver outside the warning zone.
In practice it occurs that the coordinates of a warning zone are incorrect. One cause is a human error in specifying the coordinates. The operator may for example inadvertently have entered erroneous information, or may have forgotten to update the information. Also erroneous coordinates of the warning zones may be the result of drift of the platform due to sea currents and the like. Errors in the coordinates entail the risk that a false alarm is given or even worse that no alarm is issued at all in case a vessel approaches a mooring line, so that a collision therewith cannot be avoided.